1. Field of the Invention
The present invention relates to a transmission for a bicycle provided with a derailleur.
2. Description of Background Art
In a transmission for a bicycle, a derailleur, which is simple in structure and low in cost, is used as a mechanism for shifting a chain wound around a drive sprocket and a driven sprocket for changing the speed among a plurality of sprockets provided, for example, in the driven sprocket.
For example, a derailleur for a transmission for a bicycle disclosed in Japanese Patent No. 2654101, see FIGS. 3 and 5, that includes a guide member fixed to an upper body provided in a frame of a bicycle in the axial direction and the rotational direction. A control lever is fitted to the guide member so as to be movable in the rotational and axial direction. A lower body rotatably supports a power transmission wheel which is pivotably fitted to the control lever by a first pin and on which a chain is wound. A sliding member, provided inside the cylindrical guide member so as to be capable of sliding movement, is provided with a pair of lateral pins which engage the control lever through a pair of helical slots formed on the guide member.
By moving the sliding member in the axial direction by operating a cable connected to a change lever and moving the control lever with respect to the guide member in the rotational and axial direction, the chain is shifted with respect to a plurality of the sprockets provided on a hub of a rear wheel of the bicycle.
The lateral pins are slidably fitted at both ends to a pair of the helical slots formed on the guide member move while bringing both ends into sliding contact with a pair of the helical slots when moving in the axial direction by the operation of the cable.
Therefore, a frictional resistance occurs between the lateral pins and the helical slots. More particularly, when both ends of the lateral pins which come into sliding contact with a pair of the helical slots are subjected to torque in the opposite rotational direction a sliding friction is significant. Thus, a smooth movement is impaired and a cable operation becomes heavy.